1. Field of the Invention
The present invention relates to adaptive digital filters and, more particularly, to digital filter control means.
2. Description of Related Art
The present invention is an improvement upon the invention disclosed and claimed in U.S. Pat. No. 5,014,263. For convenience and to ensure completeness of the description herein, the material in U.S. Pat. No. 5,014,263 is hereby incorporated herein by this reference thereto.
The instant invention and the invention disclosed and claimed in U.S. Pat. No. 5,014,263 both generally relate to adaptive digital filters.
As pointed out in U.S. Pat. No. 5,014,263, digital filters may be used in telephony coder/decoders, such as subscriber line audio-processing circuits (SLACs), which require periodic updating of the filter coefficients. For example, a SLAC may employ an adaptive filter to cancel echoes, that is, reflected signals, which occur if there is an impedance mismatch between transmission lines and the interconnecting SLACs.
An adaptive filter continuously adjusts its coefficients to optimize the transfer function it implements. Without the adaptation feature, a user must calculate a few sets of filter coefficients for the most commonly-encountered subscriber line characteristics and then choose the closest set of coefficients suited to the particular line used in the application. The selected set will generally not provide the best echo cancellation under any circumstances, and will generally be especially ineffective if line impedance varies over time. By employing adaptation, a user does not have to calculate any coefficients. At the same time, the user gets a continuously updated echo-cancellation function which can respond to changing line characteristics.
Digitally-encoded audio signals are commonly transmitted on networks employing SLACs. Accordingly, there is a need for adaptive digital filters which provide an echo cancellation function. Another application for digital filtering is in modulator-demodulators (modems) used for transmission of digital data over telephone lines.
Because of procedures used during the dial tone connection of SLACs or modems, the adaptive echo cancellor should be deactivated to avoid degrading the push-button signal. Furthermore, the echo controller must be deactivated if there is a "talker" at the near end of the interconnection. The adaptive filter must be prevented from updating filter coefficients during either of these circumstances. Thus, "double talker" detection is advantageously used in conjunction with an adaptive filter.
The fundamental problem addressed by the echo control circuit of telephone subscriber line cards is the mismatch in the impedance characteristics of the two wire loop and the balancing network. Because of the mismatch, some energy from the receive far-end signal is reflected back at the 2 to 4 wire hybrid and turned into an unwanted near-end echo. In order to maximize the attenuation of a near-end echo, an adaptive filter usually is employed to perform an auto-balancing function. To perform this function, the filter automatically adjusts its response matching to a varying 2 wire loop impedance through a digital adaptive process.
Certain problems can arise during operation of adaptive filters. For example, an uncontrollable or excessive adaptation of a balance filter can result in filter divergence, which is functionally undesirable. Moreover, when both near-end and far-end signals appear at same time, the adaptation will be misguided because of the merging of the near-end speech and the echo. This latter problem may be addressed by a double-talker detector. A conventional way of detecting A double-talker condition is to compare the signal level at the near- and far-ends using energy-averaging filters. If the near-end signal level exceeds the signal level of the far-end by a predetermined amount, a double-talker condition is flagged .and adaptation is inhibited.
The double-talker detector discussed immediately above has several shortcomings and deficiencies. The detector is often unreliable because the near-end speeches or signal amplitude is unknown and because the associated cable loss is also varying. This unreliability also limits the utility of the detector. A reliable double-talker detector of a line card echo controller is critical for a reliable high speed modem connection because of the duplex operation performed. This duplex operation is similar to a double-talker condition. Failure to inhibit the adaptation of an echo controller during a modem connection can result in a high bit error rate or a connection hang-up.
Based upon the foregoing, it should be understood and appreciated that .it is a shortcoming and deficiency of the prior art that there has not heretofore been developed a simple, relatively inexpensive, and reliable means for controlling an adaptive balance filter that guards against tone divergence, that operates effectively in double-talker situations, and that readily allows modem connection.